**Core Concept**
The question tests the understanding of **Froment's sign**, a clinical test used to assess the function of the **ulnar nerve**. Specifically, it evaluates the strength of the **adductor pollicis muscle**.

**Why the Correct Answer is Right**
Froment's sign is positive when there is weakness of the **adductor pollicis muscle**, which is innervated by the **ulnar nerve**. When this muscle is paralyzed, the individual will compensate by using the **flexor pollicis longus** muscle, innervated by the **anterior interosseous branch of the median nerve**, to adduct the thumb.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the muscle responsible for Froment's sign is not primarily involved in thumb opposition.
**Option B:** Incorrect as the muscle involved in Froment's sign is specific to the ulnar nerve, not the median nerve.
**Option C:** Incorrect because while the adductor pollicis is involved, the specific option provided does not match the correct muscle.

**Clinical Pearl / High-Yield Fact**
Froment's sign is a crucial clinical test for assessing ulnar nerve function, particularly in cases of suspected ulnar nerve damage or **cubital tunnel syndrome**.

**Correct Answer:** D. Adductor pollicis muscle